A hunter likely sparked the monster bushfire that spread into America's world-renowned Yosemite National Park and became the fourth biggest blaze in California's history, officials say.

They dismissed earlier reports that the so-called Rim Fire, which is now 80 per cent contained, was caused by activity on an illegal marijuana farm near the US landmark park.

"Investigators from the US Forest Service Law Enforcement and Investigations and Tuolumne County District Attorney's Office have determined the Rim Fire began when a hunter allowed an illegal fire to escape," a statement by the US Forest Service said.

"There is no indication the hunter was involved with illegal marijuana cultivation on public lands and no marijuana cultivation sites were located near the origin of the fire.

"No arrests have been made at this time."

The hunter's name is being withheld pending further investigation, the statement added.

A hunter allowed an illegal fire to escape.

US Forest Service

The fire, which began on the afternoon of August 17 in the Stanislaus National Forest just outside Yosemite and is named for a popular local lookout spot called Rim of the World, now covers 95,442 hectares, according to the latest update on the Inciweb inter-agency website.

Firefighting costs there have run to $81 million to date, with more than 4,300 firefighters still working to contain the blaze, while aircraft have dropped more than 15 million litres of water and fire retardant over the past 17 days.

Fire managers say they expect the blaze to continue burning for several more weeks, and warned that a resumption of gusty winds "could challenge containment lines".

Several campgrounds, numerous trails and one of the four main entrances to the park have been closed by the fire. But the most popular portions have remained open to the public, including the scenic Yosemite Valley area famed for its towering granite rock formations, waterfalls and meadows.

Nevertheless, park officials say droves of visitors who typically crowd Yosemite in late summer have noticeably diminished. And Yosemite-area businesses say the region's tourist economy has been hit hard a year after an outbreak of hantavirus in the park frightened away many park visitors.

Authorities in California have in recent years faced increasing problems with marijuana farms hidden deep in the region's rugged wilderness.

A 2009 fire that burned 36,421 hectares in the Los Padres National Forest near Santa Barbara was triggered by a campfire at a marijuana farm.

The current blaze is the fourth largest California bushfire since records began 1932, and covered an area five times that of Washington, DC.

The largest in California history remains the 2003 Cedar fire in San Diego County, which destroyed 2,820 buildings and left 14 people dead after ripping through 110,579 hectares of land.